Conventionally, there is known a plasma processing apparatus for generating a plasma in a processing chamber by using a high frequency power generated by a high frequency power supply. In addition, there is a technique for generating a plurality of high frequency powers having different frequencies by using a plurality of high frequency power supplies (see, e.g., Japanese Patent Application Publication No. 2012-015534).
However, in the case of generating the plasma in the processing chamber by using the high frequency power generated by the high frequency power supply, it is difficult to stably maintain the plasma under an environment of a low pressure and a low plasma density. For example, in order to maintain the plasma under a low pressure environment, it is considered to increase the high frequency power generated by the high frequency power supply. However, when the high frequency power is increased, an electric field in the processing chamber is increased. Accordingly, ionization of the plasma is accelerated and, thus, the plasma density may be excessively increased.
In order to suppress the excessive increase in the plasma density, it is considered to decrease the high frequency power to a level smaller than that at the time of plasma generation (ignition) after the plasma generation (ignition). However, when the high frequency power is decreased, the electric field in the processing chamber is decreased. Therefore, the electric field that is enough to maintain the plasma is not ensured. As a consequence, the plasma may be lost. From the above, a low frequency is preferred in order to obtain high ion energy by a conventional CCP (Capacitively Coupled Plasma) type plasma apparatus. Since, however, ion energy distribution becomes broader, it is difficult to accurately control the ion energy by suppressing the ion energy distribution.